Method of producing from wood tar a liquid smoke product for use in food processing, and product of said method

ABSTRACT

A liquid smoke product which contains no detectable amount of 3,4-benzopyrene (also known as benzo(a)pyrene) consists essentially of propylene glycol together with those components of the heavy, essentially water insoluble material that settle out of an aqueous condensation of wood smoke (hereinafter called &#34;wood tar&#34;) which codistill at or below the boiling point of propylene glycol and most of which are soluble in propylene glycol. 
     A method of making the above product by mixing about three parts by weight of wood tar with one part by weight propylene glycol to form a feed material, heating said feed material to about 280° F. to 290° F., reducing the pressure to about 100 mm. of mercury to cause codistillation of the desired components with the propylene glycol and thereby form a liquid fraction and a vapor fraction, entirely separating the vapor fraction from the liquid fraction, and condensing the vapor fraction which is then treated with a small percentage of activated carbon to assure that the amount of 3,4-benzopyrene is below 0.5 parts per billion, by weight.

BACKGROUND OF THE INVENTION

Several types of liquid smoke products are known in the food processingindustry. One well-known type is an aqueous smoke solution which isproduced in accordance with the method of Hollenbeck U.S. Pat. No.3,106,473. Such an aqueous smoke solution is relatively light in smokeflavor, and is usable only in the processing of foods with which wateris compatible. Hollenbeck U.S. Pat. No. 3,480,446 discloses a method ofpreparing a smoke flavored edible oil. A smoke flavored oil produced bythe method of patent 3,480,446 also has a rather light smoke flavor,because its starting material is the aqueous smoke flavored solution ofpatent 3,106,473.

When wood smoke is extracted in water, or in the presence of water,there is a substantial heavy fraction of essentially water insolublematerial. That material is herein referred to as "wood tar", and theterm "wood tar" will be used throughout this specification and claims asthe name for that material, which is a very complex mixture of organiccompounds. It has heretofore been considered to be a waste product, andis one which presents a severe disposal problem.

SUMMARY OF THE INVENTION

In accordance with the present invention, the wood tar is mixed in theproportion of between about three parts by weight and twenty parts byweight of tar to about one part by weight of propylene glycol to form afeed material, the proportions depending upon the consistency of thetar; the feed material is maintained constantly in motion while it isheated to a temperature of about 280° F. to 290° F.; and it is thensubjected to a reduced pressure, such as 100 mm. of mercury. Thisvaporizes the propylene glycol and all of those components of the woodtar which codistill at or below the boiling point of the propyleneglycol at the reduced pressure. The vapor fraction is conducted awayfrom the liquid fraction through a demister comprising a material suchas stainless steel wool which traps entrained liquid. The vapor fractionis then condensed and intimately contacted with a small percentage ofactivated carbon to assure that it contains less than 0.5 parts perbillion of 3,4-benzo-pyrene.

Practical commercial processing does not permit complete separation ofthe wood tar from the water and the smoke components dissolved in it, sothe raw material for the practice of this invention may contain about10% water and water soluble smoke components. Accordingly, "wood tar"for purposes of this invention may include a small percentage of aqueoussmoke.

The liquid smoke product of the present invention consists, therefore,of propylene glycol, together with those wood tar components whichcodistill under reduced pressure, such as 100 mm. of mercury, at orbelow the boiling point of propylene glycol. Most of such components aresoluble in propylene glycol. In addition, the product contains a smallamount of water and water soluble smoke components.

The product has a very high level of smoke flavor (40 to 50 timesstronger than aqueous smoke) so that only a very small quantity of it isrequired to flavor a rather substantial amount of a food product. Aftertreatment with about 1% by weight of powdered activated carbon theproduct contains no detectable amount of 3,4-benzopyrene as determinedby very sensitive chromatographic analysis. A method of analysis whichwill disclose the presence of as little as 0.5 parts per billion of3,4-benzopyrene in the product is described in "Determination ofBenzopyrene in Smoke Condensates by High Pressure Liquid-LiquidChromatography", Journal of Food Science, Vol. 39 (1974). Accordingly,the phrase "no detectable amounts of 3,4-benzopyrene" is used herein tomean that it is not detectable by the above described analysis.

While propylene glycol is the preferred material for use in the methodof the present invention, it is apparent that other edible liquids, suchas butylene glycol, which have a sufficiently low boiling point andwhich are solvents for all or most of the components of the wood tar maybe used in the practice of the method of the present invention.

Obviously, any edible liquid to be used in the present method mustimpart no undesirable characteristics to the smoke product, and must becompatible with any food products with which the material may be used.

The word "codistillation" is used herein to define the simultaneousvaporization of those components of a mixture which can be vaporized ator below a selected temperature, even though some of those componentsmay have a boiling point or boiling range which is above the selectedtemperature.

THE DRAWING

The drawing is a diagram of the apparatus and processing steps used inthe method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As previously indicated, the present method makes a commerciallyacceptable product out of the wood tar which is a discarded byproduct inthe production of certain types of aqueous smoke. The raw material forthe practice of the present method may, therefore, be produced by any ofseveral commercially practical methods for producing an aqueous smokeproduct and wood tar.

The wood tar, usually together with about 10% of aqueous smoke solution,provides the raw material for the practice of the present invention. Itis heated to about 140° F. to make it flowable, and is introduced into amixing tank where it is thoroughly mixed with propylene glycol to form afeed material. When the raw material is processed with propylene glycol,a satisfactory ratio is about three parts by weight of raw material toone part by weight of propylene glycol. This ratio, however, may varyover a rather wide range depending upon the characteristics of the rawmaterial and the method by which it is produced. Based upon the workperformed by the inventors, but without having tested the entire rangespecifically, it is believed that as little as one part by weight ofpropylene glycol to twenty parts by weight of the raw material can beused to obtain a satisfactory product. The maximum ratio of propyleneglycol to raw material is limited principally by economicconsiderations.

By the time the feed material is ready to be pumped out of the mixingtank its temperature is reduced to about 75° F., and a feed pump movesit into a processing system, at least a portion of which is maintainedbelow atmospheric pressure by a vacuum pump. Although the vacuum pump iscapable of maintaining a pressure of about 100 mm. of mercury in thesystem, the pressure at the feed pump is substantially higher than thisbecause of the relatively small diameter pipe between the feed pump andthe two heat exchangers in the system, and a restricted orifice at theexit of the second heat exchanger.

The system includes two plate-type heat exchangers, each of which has aninternal structure of heat exchange baffles which produce considerableturbulence in the feed material. The feed material is preheated in thefirst heat exchanger to a temperature of about 220° F. and is thenheated in a second heat exchanger to about 280° F. to 290° F.

From the second heat exchanger the feed material enters a low-pressurechamber. The pressure inside the chamber is about 100 mm. of mercury sothat the feed material is rapidly separated into a vapor phase at atemperature of about 220° F. to 230° F., and a liquid phase at atemperature of 270° F. to 280° F. From the low-pressure chamber thevacuum pump draws the vapors through a demister comprising stainlesssteel wool which removes fine particles of entrained liquid from thevapor. The vapor, of course, includes most of the propylene glycol, thecomponents of the wood tar which codistill at or below the temperatureof the vapor at the pressure which prevails in the low pressure chamber,together with water vapor and vaporized water soluble smoke components.

It will be readily apparent to those skilled in the art that theseparation process which takes place in the low pressure chamber is asingle stage codistillation of propylene glycol (or other solvent) andthose components which codistill at or below the vapor temperature inthe chamber. This temperature is effectively controlled by the selectionof the solvent. Propylene glycol is especially suitable in the presentcase because its boiling point at or below atmospheric pressure issubstantially lower than that of the undesirable3,4-benzopyrene-containing substances present in the wood tar. Thus,most of these undesirable substances remain in the liquid as the solventand the desirable, lower boiling-point components vaporize and areremoved.

A low pressure chamber is used to prevent thermal decomposition ofcertain components of the wood tar which occurs at or close to theboiling point of propylene glycol at atmospheric pressure, and also toreduce energy requirements. The pressure in the chamber should be lessthan about 2/3 atmosphere, and about 100 mm. of mercury is desirablebecause it can be achieved with a relatively inexpensive, standard watersealed vacuum pump.

From the low-pressure chamber the vapor is drawn into heat exchangetubes of a conventional shell-and-tube condenser which is cooled bywater which enters the condenser jacket at about 64° F. and is exhaustedfrom the condenser jacket at about 72° F. The condensed product, atabout 80° F., is withdrawn from the discharge end of the condenser by acondensate removal pump.

The final step of the present invention consists of the adsorption ofcertain undesirable components, including any traces of 3,4-benzopyrene,from the liquid smoke. As schematically illustrated in the drawing, theadsorption is preferably effected by intimately contacting the liquidsmoke with a controlled proportion of activated carbon. After theadsorption step there is less than 0.5 parts per billion, by weight of3,4-benzopyrene in the product.

Activated carbon is commercially available in granular form and inpowder form. The latter form is preferred because it does not trap andhold the feed material and thus affords much higher yields than areproduced with the granular material.

One example of a commercially available activated carbon is "Type PWAPulverized Carbon" produced by Pittsburgh Activated Carbon Division ofCalgon Corporation, Pittsburgh, Pennsylvania. Type PWA Pulverized Carbonhas a particle size of 65-75% minus 325 mesh (U.S. Sieve Series, openingof 44 microns), and a total surface area (N₂ B.E.T. Method) of 1000-1100m² /g. This type of carbon has been pre-washed with acid to removeiron-containing contaminants. It is known that iron contaminants presenton the carbon can form complexes with certain phenolic components of theliquid smoke, thereby imparting an undesirable dark color to the finalproduct.

The contacting of the activated carbon and the liquid smoke ispreferably effected by agitation in conventional mixing equipment. It isnecessary that the agitation continue only so long as to allowadsorption of any 3,4-benzopyrene present in the liquid smoke. While therequired period of time may vary with the percentage of activated carbonto liquid smoke, it has been found that an agitation time of 15 minutesis sufficient to remove all detectable amounts of 3,4-benzopyrene fromliquid smoke when 1% by weight of activated carbon powder is mixed withthe liquid smoke. Larger quantities of activated carbon may be used but1% by weight is sufficient to adsorb all detectable amounts of3,4-benzopyrene without causing a noticeable change in the flavorstrength of the product. As progressively larger amounts of activatedcarbon are used yields are reduced and somewhere between about 5% and10% there are detectable diminutions in flavor strength.

The carbon-liquid mixture is then filtered to remove all of the solidconstituents from the mixture. Filtering may be done in the conventionalmanner, as with a vacuum drum filter or through a filter-aid such asdiatomaceous earth, for example. Filtering removes trapped particulateswhich may have been carried through the demister, as well as theactivated carbon. The resulting product may then be diluted or otherwiseprepared for commercial use.

A residue pump removes the spent wood tar from the bottom of the lowpressure chamber at about 270° F. to 280° F., and the tar is depositedin drums or other containers for disposal.

The vacuum pump which reduces the pressure in the system to about 100mm. of mercury may, under certain conditions, suck minute particles ofcondensate from the discharge end of the condenser. To prevent any suchparticles from entering the vacuum pump and being discharged to theatmosphere, the vacuum line may pass through an entrainment separatorbetween the discharge end of the condenser and the vacuum pump. Theunlikelihood of such entrainment occurring, and the small quantities ofentrained material involved if it does occur, make the use of anentrainment separator at this point of marginal value, even for thepurpose of avoiding air pollution. The entrainment separator has novalue in the economics of the system.

As previously indicated, although propylene glycol is the preferredsolvent, other edible liquids such as butylene glycol may be used in thepractice of the method of the present invention. The use of a solventwhose boiling point differs from that of propylene glycol may slightlyalter the flavor and aroma characteristics of the final product becausethe codistillation temperature depends upon the solvent, and alterationof this temperature results in the addition or deletion of certain woodtar components in the vapor fraction.

A specific example will provide an illustration of the practice of ourinvention.

A feed mixture was prepared by mixing one part by weight of propyleneglycol with three parts by weight of the water insoluble wood tarrecovered from the product of absorption of wood smoke by water, asdescribed above. The wood tar-propylene glycol mixture was pumped atapproximately 600 lbs./hour through a first heat exchanger wherein themixture was preheated to approximately 220° F. The preheated feed thenentered another heat exchanger where it was further heated to about 290°F. so as to vaporize a substantial portion of the mixture.

The liquid-vapor mixture passed from the second heat exchanger to aliquid-vapor separator of the conventional type, which was maintainedunder a vacuum, with the total pressure being approximately 100 mm. ofmercury. The non-vaporized liquids, at a temperature of about 270°F.-280° F., were removed from the separator for disposal at a rate ofabout 400 lbs./hour. The vapors, at a temperature of about 220° F.-230°F., passed from the separator at a rate of about 200 lb./hour through a"packed" exit or "demister" of the conventional type, wherein entrainednon-vaporized tar constituents were removed.

The vapors passing from the demister entered a conventionalshell-and-tube condenser wherein they were condensed by heat exchangewith cool (65° F.) water and cooled to approximately 80° F.

The condensate was mixed with 1% by weight of Type PWA Pulverized Carbonand agitated for 15 minutes. The mixture was then filtered through alarge Buchner-type filter which was pre-coated with a filter aidcomprising diatomaceous silica and marketed under the trade name"Celatom FW-14" by Eagle Picher Products. The filter aid was pre-washedwith acid to remove iron contaminants to prevent undesirable colorationof the final product, as discussed above in connection with theactivated carbon.

The filtrate contained less than 0.5 parts per billion, by weight, of3,4-benzopyrene, and no change in flavor or aroma chacteristics could bedetected. The filtrate was then mixed with propylene glycol tostandardize the final product to about 18% smoke constituents beforefinal packaging and shipment. Such dilution may alternatively take placebefore the carbon mixing.

The foregoing detailed description is given for clearness ofunderstanding only and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

We claim:
 1. A method of treating wood tar to make a liquid smokeproduct which contains no detectable amount of 3,4-benzopyrene, saidmethod comprising:(a) forming a feed material by mixing from a maximumof about twenty parts by weight to about three parts by weight of woodtar with one part by weight of propylene glycol; (b) heating said feedmaterial to about 220° F. followed by heating said feed material toabout 280° F., said feed material being agitated while being heated; (c)subjecting said feed material to a pressure less than about 2/3atmosphere whereby said feed material is separated into a vapor fractionconsisting of said propylene glycol and the components of said wood tarwhich codistill at or below the boiling point of the propylene glycol,and a liquid fraction; (d) separating the vapor fraction from the liquidfraction; (e) condensing said vapor fraction; (f) intimately contactingsaid condensed vapor fraction with pulverized activated carbon to assurethat no detectable amount of 3,4-benzopyrene is present in said vaporfraction; and (g) filtering said activated carbon powder from saidcondensed vapor fraction.
 2. The method of claim 1 in which thepulverized activated carbon has a particle size of 65-75% minus 325 mesh(U.S. Sieve Series, opening of 44 microns), and a total surface area (N₂B.E.T. Method) of 1000-1100 m² /g.
 3. The method of claim 1 in which thepulverized activated carbon has been pre-washed with acid to eliminateiron contaminants capable of reacting with components of the liquidsmoke.
 4. The method of claim 1 wherein said adsorbent is removed fromsaid condensed vapor fraction by filtration.
 5. The liquid smoke productproduced according to the method of claim 1.